The present invention relates to a process for manufacturing glass optical elements such as glass lenses, which require no grinding and polishing after their press molding. In particular, the present invention relates to a process for manufacturing glass optical elements, which can improve production speed by markedly shortening the cycle time required for the press molding.
There have been known various processes for manufacturing glass optical elements, which require no grinding and polishing after their press molding, by molding glass preforms, which are glass materials to be further molded, in molds which ensure surface accuracy and surface roughness required for surfaces of molded glass articles.
For example, Japanese Patent Un-examined Publication (KOKAI, hereinafter referred to as "JP-A") No. 64-72929 and Japanese Patent Publication (KOKOKU, hereinafter referred to as "JP-B") No. 2-16251 disclose processes where molds and glass preforms are heated together. In these methods, a glass preform is inserted into a mold assembly comprising an upper mold, a lower mold and a guide mold which guides the upper mold and the lower mold, and the preform is heated together with the mold assembly to a temperature where the preform is softened sufficiently and then the preform is subjected to press molding. Then, they are cooled at such a cooling rate that surface accuracy of the glass article after molding is not deteriorated to a temperature around the glass transition temperature, or allowed to cool to room temperature with a certain period of time, and the molded glass article is removed from the mold assembly.
JP-A-62-113730 and JP-B-63-46010 disclose processes where a preliminarily softened glass preform is inserted into a separately heated mold assembly. In these methods, a glass preform placed on a ring-like member is softened by heating it together with the ring-like member, inserted into a mold assembly with the ring-like member and press molded between an upper mold and a lower mold which penetrates the ring-like member and lifts up the softened preform. Alternatively, the ring-like member acts as a guide mold guiding the upper mold and the lower mold to perform the press molding. JP-A-61-251529, JP-A-61-286232, JP-A-62-27334 and JP-A-63-45134 also disclose processes for molding glass optical elements where a preliminarily softened glass preform is inserted into a separately heated mold assembly. However, these processes have drawbacks that they occasionally cannot mold a desired shape when relatively large deformation of the glass material is required, and that they are likely to generate sink marks and distortion and thus difficult to obtain sufficient surface accuracy.
JP-A-62-27334 discloses a process where a glass preform is inserted into a mold assembly by using a ring-like member and molded, as well as temperature conditions for prolonging the lifetime of molds for such a process. In this method, the mold temperature is maintained within a temperature range of from a temperature just below the glass transition point to a temperature lower than the glass transition point by 200.degree. C., and a glass preform, which has been preliminarily heated to such a temperature that the preform had a viscosity ranging from 10.sup.6 to 10.sup.8 poises, is inserted into the mold assembly and press molded.
In the above-mentioned processes where a preform is heated, molded and cooled with molds while the preform is maintained in a mold assembly, the temperatures of the glass and the molds are approximately the same throughout the molding process and hence there would be no temperature difference between the surface and the inside of the glass. Therefore, sink marks are prevented and thereby high surface accuracy is provided. However, since it requires a temperature elevating period before the pressing and a cooling period after the pressing and before the ejection, it has a drawback of extremely long cycle time required for the whole process. In addition, since the glass is contacted with mold surfaces for a long period of time during the heating and the pressing, it has also a drawback that the glass is likely to react with the mold surfaces and thereby the lifetime of the molds is shortened.
On the other hand, in the process where a glass preform which is preliminarily heated to have a higher temperature (low viscosity) than molds is inserted into the molds by using a ring-like member and press molded, press time may be very short. In addition, since the mold temperature may be relatively low and release of a molded glass from molds is possible after a relatively short period of time to allow the molded glass to cool after the pressing, the cycle time can be markedly shortened. However, if the preform is inserted into the molds at a low temperature (within a temperature range of from a temperature just below the glass transition point to a temperature lower than the glass transition point by 200.degree. C.) to prolong the lifetime of the molds, the temperature of the glass surface is rapidly lowered and the glass is cooled and solidified before it is press molded to a desired thickness. Therefore, it has drawbacks that it cannot stably provide molded articles, especially glass molded articles with a small edge thickness (about 1.0 to 1.3 mm) such as biconvex lenses and meniscus lenses, and that it shows insufficient surface accuracy.
To solve the above-described problems, it has been proposed to use a glass preform showing a further lower viscosity under similar temperature conditions of molds. However, as the viscosity becomes lower, the softened preform on the ring-like member becomes more likely to sag at the opening of the ring member (deformed and hanged down). For example, though it depends on the shape of the preform, when the viscosity is 10.sup.7 poises or lower, the preform is very likely to sag. Therefore, to prevent deformed preform from dropping down from the ring-like member, it is necessary to use a glass preform having an outer diameter quite larger than the inner diameter of the preform supporting portion of the ring-like member. As a result of it, press molded lenses have a quite larger outer diameter than desired and hence it is necessary to cut off a large surplus in a post-processing so that they have a desired outer diameter. Further, in this method using a ring-like member, since molding flash is produced due to the use of a preform larger than the final product and the generation of surplus and, since a low mold temperature is used, it is very difficult to produce biconvex lenses, meniscus lenses and the like with a small edge thickness.
Therefore, one of the objects of the present invention is to provide a process for manufacturing glass optical elements by press molding a heated and softened glass material such as a glass preform in preheated molds, wherein the glass material is easily held during its heating and softening even if a glass material such as a glass preform of which viscosity is decreased when it is softened and hence which is likely to deform is used and thus a glass optical element can be produced.
A further object of the present invention is to provide a process capable of satisfactorily manufacturing glass optical elements by transferring a heated and softened glass preform and the like, which is prone to be deformed, to molds without unduly deforming it.
A further object of the present invention is to provide a process for manufacturing glass optical elements, which uses a glass material such as a glass preform which enables to provide a molded glass with a size approximately the same with an effective outer diameter desired for a purpose glass optical element and therefore can minimize an edging volume for centering in a post-processing.
A further object of the present invention is to provide a process for manufacturing glass molded articles of which cycle time required for press molding is remarkably shortened and which can provide glass molded articles with no surface defects and with high surface accuracy.
An additional object of the present invention is to provide a process capable of easily manufacturing biconvex lenses, meniscus lenses and the like with a small edge thickness.
A still further object of the present invention is to provide a process capable of transferring a heated and softened glass gob, which is prone to be deformed, to molds to satisfactorily manufacture glass optical elements.
One of the objects of the present invention is to provide a process for manufacturing glass optical elements by press molding a heated and softened glass material such as a glass preform in a preheated molds, which can remarkably shorten the cycle time required for the press molding, stably provide lenses and the like even though they must have a small edge thickness and show good surface accuracy.
A further object of the present invention is to provide a process for manufacturing glass optical elements without sink marks and distortion and with high surface accuracy.
A further object of the present invention is to provide a process capable of manufacturing glass optical elements without sink marks and distortion and with high surface accuracy and a center thickness within an allowance.